VW Hydri

VW Hydri is a superb cataclysmic variable for southern observers, and belongs to the interesting subclass of dwarf novae known as SU Ursae Majoris stars, or UGSUs. The UGSUs have two different kinds of outburst behaviors: normal outbursts lasting for a few days and having moderate amplitudes, and superoutbursts being typically a magnitude brighter at maximum, and lasting for 2-3 weeks. VW Hyi is a fine example of this class, with one of the better defined outburst and superoutburst recurrence times. Normal outbursts occur about once a month on average, while superoutbursts normally occur twice yearly. Like many other UGSUs, VW Hyi also shows an interesting short-period modulation in its light curve known as a superhump. These oscillations are due to precession of the accretion disk around the white dwarf; the period of the superhump is a beat period of the orbital period (typically under two hours), and the precession period of the disk (a few days). The superhump period winds up being a few percent longer than the orbital period.

VW Hyi has been a frequent target of both amateur and professional observers, and AAVSO observers have often paired with the professional community to assist with major observing campaigns by the Hubble Space Telescope, SAX, and the Hopkins Ultraviolet Telescope.